1. Field of the Invention
This invention relates generally to the production of hydrocarbons from underground hydrocarbon bearing formations by in situ combustion processes.
2. Description of the Prior Art
In order to produce hydrocarbons from permeable underground hydrocarbon bearing formations it is customary to drill one or more boreholes or wells into the hydrocarbon bearing formation and produce the hydrocarbons such as oil through designated production wells either by the natural formation pressure or by pumping the wells. At some point, however, the flow of hydrocarbons diminishes and may even cease even though substantial quantities of hydrocarbons are still present in the underground formations. In other cases the petroleum in the reservoirs is so viscous that it will not initially flow to production wells.
Thus, secondary recovery programs are now an essential part of the overall production planning for every petroleum reservoir in underground hydrocarbon bearing formations after primary production has ceased to be economical. In general, secondary recovery operations involve injecting an extraneous fluid such as water or gas into the hydrocarbon bearing zone to drive the petroleum toward production wells by the process frequently referred to as flooding. Other methods of secondary recovery or even primary recovery involve heating for the recovery of hydrocarbons from the subterranean formation, particularly containing viscous crude and tar sand bitumen. When heated, the hydrocarbon material in the formation becomes less viscous or the chemical composition of the hydrocarbon material is changed to form a material which has a lower viscosity. In either case, the hydrocarbon material in the formation is able to flow more readily through the formation and is recovered from a production or output well. One of these thermal recovery methods involves the combustion of a portion of the hydrocarbon material within the formation. In this method an oxidizing gas (normally oxygen in the form of air) is passed into the formation through an input or injection well and the hydrocarbon material within the formation is ignited by suitable means. This oxidizing gas may be referred to as a combustion supporting gas. The zone of combustion or combustion front produced by the ignition migrates through the formation and the hydrocarbons displaced from the formation by the traveling combustion front are driven through the formation in the direction of the output well. The hydrocarbons enter the output well and they are removed therefrom and brought to the surface of the earth. While this method, termed the in situ combustion method, is satisfactory from the standpoint of the results desired it is subjected to certain drawbacks.
Due to the heat generated within the formation by the in situ combustion process, oxidation reactions at moderate to low temperatures may continue to occur behind the combustion front. This oxidation is a result of injected oxygen in air contacting some of the remaining crude oil or coked hydrocarbons which are usually found behind the combustion front. This reduces the concentration of the combustion supporting gas (oxygen) before it is able to reach the leading edge of the combustion zone or high temperature front. The region behind the front is usually heated to moderate temperatures by the passing combustion front and some areas of this region retain a sufficient amount of hydrocarbons to oxidize a great deal of the combustion supporting gas at these moderate temperatures even after the combustion front has passed. The combustion supporting gas is normally oxygen which may be contained in air or derived from any other source desired.
For a given oxygen injection flux the oxygen concentration reaching the combustion front decreases as the distance from the injection well to the front increases. Inevitably at some distance from the injection well the oxygen flux at the front becomes too small to sustain combustion and the combustion front velocity becomes negligible. At this point the in situ combustion process becomes inefficient as oxygen injection is wasted by the low temperature oxidation and oil is no longer displaced. This condition is reached as the temperature at the front decreases to below a temperature where combustion efficiency for displacing petroleum becomes negligible. As the velocity of the combustion front decreases, the tendency in the past has been to increase the amount of oxygen injected in order to maintain a higher oxygen flux and increase the velocity of the combustion front. However, this soon becomes uneconomical at the higher injection pressure as the ever widening heated combusted zone behind the front consumes more and more injected oxygen. A condition of diminishing returns in the form of an even slower moving combustion front steadily overtakes the injection well's capacity to handle the increased pressure and volumes of oxygen. Also, since more and more oxygen is needed which may not even maintain the velocity of the combustion front, the economics of the in situ combustion process steadily deteriorates and soon becomes prohibitive.
Our invention overcomes these drawbacks by providing for an in situ combustion process with a plurality of combustion supporting gas injection wells spaced so as to anticipate the maximum distance a combustion front will travel. The term "injection wells" as used herein means a geographical injection center containing one or more actual wells or completions equidistant from the injection center.